The relationship of Muhammad with his Lord was based on complete faith and absolute sincerity, true love, obedience, submission, and surrender to Him (SWT). If we summarize and describe this very special relationship between Muhammad and his Lord in one word, we will not find a better word than the word "Islam".
Yes, this relationship was complete and total Islam (surrender) to Allah (SWT)! This is the type of relationship which Allah (SWT) desires between Him and His worshippers. Muhammad embodied this relationship even before the Revelation. Allah (SWT) desired that this example widespread to all people, so He (SWT) sent Muhammad with it to become a religion and way of life.
Religion, by definition is nothing but a set of guidelines that define the relationship between God and His servants. He (SWT) named this religion Islam, to summarize that type of relationship which was started by Abraham (AS) who was praised in the Qur’an in the following verse,
“Abraham was indeed a model, devoutly obedient to Allah, (and) true in Faith, and he was not of the polytheists.” [16:20]
The prophets after him tread that path, and then it was completed, perfected, and lived by Muhammad (AS) in the best, purified, and complete form. It is as if Allah (SWT) wants to tell His servants that whoever of you wants to have a religion with me (meaning to have a relationship with me), this relationship should be complete surrender (Islam), as Abraham (AS) the Friend began and was concluded by Muhammad - the Chosen one, on the same basis and foundations.
As for those who seek different way to achieve proximity with Allah (SWT), it will not be accepted from him. Allah (SWT) says in the Qur’an,
“Surely the (true) religion with Allah is Islam [3:19].”
He (SWT) also says,
“If anyone desires a religion other than Islam, it shall not be accepted from him, and in the hereafter he shall be one of the losers [3:85].”
So the relationship of Muhammad with his Lord before Islam was in essence Islam (submission). Allah (SWT) loves it, so He (SWT) chose Muhammad (SWT) to spread this Message and make it a path and religion for all of mankind. Allah (SWT) designated Muhammad as the messenger and prophet of Islam; Muhammad would invite the people to it because he already practiced it, mastered it, and was sincere in it.
So Prophet Muhammad (SA) is most worthy of being assigned this job after Allah (SWT) completed and perfected the message with the rules, verses, jurisprudence, obligations, restrictions, and the Holy Book. There is no prophet who didn’t call for Islam as a way for the relationship between the Creator and His creation, and to become a religion by which they worship Allah (SWT) who will be pleased of those who worship Him through this way and this approach.